1. Technical Field
This invention relates to power adapters, and more specifically, to a system and method to correct power factor, i.e., the ratio of real power to apparent power.
2. Description of the Related Arts
The explosive growth in consumer electronics is causing the electricity supply industry considerable concern. The appliances or consumer electronics devices employ power supplies that draw current from the AC power line during the peak of the sine wave. Most of the appliances or consumer electronics device utilize a rectifier-bridge/smoothing capacitor circuit.
Power factor is the ratio of real power to apparent power. In the United States, power is provided at approximately 120 Volts AC with a frequency of approximately 60 Hertz. In Europe and other areas, power is provided at approximately 240 Volts AC with a frequency of approximately 50 Hertz. In order to provide a maximum amount of usable energy or power, it is desirable for a load to draw current as if the load is entirely resistive. If the load appears resistive, then the current drawn from the source may have a substantially sinusoidal shape, as the AC voltage has, and the current drawn from the source may be in phase with the AC input voltage.
Power supplies that utilize rectifier-bridge/smoothing capacitor circuits draw non-sinusoidal currents as the AC line's instantaneous voltage exceeds the storage capacitor's voltage. The electricity generator, with no power factor correction, must supply energy at the top/peak of the sine wave rather than throughout the cycle, which can cause the sine wave to collapse around its peak.
The electricity generator sees the phase lag between the current and voltage, together with the harmonics from peaky loads, as combining to provide require rms currents, which in turn reduces the real power that the network can supply. Varying loads at the consumer end of the line produces fluctuations throughout the local line and these fluctuations cause undesirable consequences, such as causing lighting sources to flicker.
FIG. 1 illustrates the current and voltage waveforms for an electronic device that power factor correction (PFC) is designed to correct according to the prior art. As illustrated, the voltage waveform is sinusoidal in shape and the current waveform can be characterized as a waveform with a steady current value with large spikes in the amplitude of the current waveform along with a high content of harmonics. The large spikes in the current waveform are caused because of the switching power supplies' use of the rectifier bridge/smoothing capacitor circuits. From an efficiency viewpoint, a typical uncorrected switched-mode power supply has a power factor of 0.6, which effectively reduces the current available from the AC socket from about 13 to about 7.8 Amps.
A solution for power factor correction is to condition the equipment's input load power so that it appears purely resistive using active PFC techniques. Common PFC designs employ a boost preconverter ahead of the conventional voltage-regulation stage, which effectively cascades to switched-mode power supplies. The boost preconverter raises the full-wave rectified, unfiltered AC line to a DC input rail at a level slightly above the rectified AC line, which is typically around 375 to 400 volts DC. By drawing current throughout the AC line cycle, the boost preconverter forces the load to draw current in phase with AC line voltage, quashing harmonic emissions.
FIG. 2 illustrates a power factor correction circuit with a boost preconverter according to the prior art. The full-wave bridge rectifier 200 receives the AC input voltage and produces a full-wave rectified voltage. The boost preconverter 205 receives the full-wave rectified voltage and forces the load to draw current in phase with the voltage. The shape of the current waveform is determined by a switching device 215, which is coupled to the output and a control circuit 220. The control circuit 220 provides an input to the switching device 215 and receives as input signals a signal from the output and a signal from the rectifier/boost node 225. This circuit may solve the power factor problem by shaping the current waveform to mimic the voltage waveform and to cause the current waveform to be in phase with the voltage waveform. However, the circuit utilizes at least five diodes, four of which are located in the bridge rectifier, and diodes are lossy components, which decreases the power efficiency of the circuit.
Accordingly, it would be beneficial to have fewer lossy components in a power factor correction circuit, where the power factor correction circuit accepts a wide range of input voltages and automatically adjusts the current waveform provided to be substantially sinusoidal in shape and in phase with the AC input voltage waveform.
It would also be beneficial to utilize the circuitry that is rectifying the AC input voltage to assist in providing power factor correction.